A steam turbine, as described in US patent application no. 2011/0103970A1, may comprises a rotor with an stress relief piston comprising a relief groove for relieving thermal stress that is outside the region of the live steam flow path that is displaced axial opposite the direction of the operating steam flow through the blade flow path.
With the increased use of renewable power there is an increased need for electric network operation to operate with increased cycling. This increase in operational flexibility may typically be limited by the steam turbine life as increased exposure to frequent thermal transient's increase the risk of the occurrence of thermal fatigue crack initiation during cold, warm and hot start-ups as well as during shutdowns. While this problem may be partially addressed through high quality rotor forgings that improved toughness and ductility, however, these measures do not overcome the negative effects thermal transients have on low cycle fatigue life of the rotor.
An additional problem is that in steam turbines having steam turbines, for example a high pressure turbine and an intermediate pressure turbine, different thermal conditions in each of the steam turbines results in different low cycle fatigue life of rotor portions of each of the steam turbines. The result can be unsynchronised maintenance schedule requirements of each of the steam turbines which may result an increase in maintenance outages. Although it may be possible to balance the low cycle fatigue life of rotor portions by the selection of rotor materials, there are practical limitations on achieving the objections by with rotor material selection alone.
There is therefore a need to both improve the low cycle fatigue life of steam turbine rotor portions as well as tailor the low cycle fatigue life of different portions to synchronise rotor portion maintenance cycles.